Tapered-pole-face generator



C. A. AWM/ELL AL TAPERED-PGLE-FACE GENERATOR Filed Nov. 20 1947 lmin! Wolfs March 22, 49

Load Hmeres Patented Mar. 22, 1949 TAPERED-POLE-FACE GENERATOR Clarence A. Atwell and Winston A. Brecht, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 20, 1947, Serial No. 787,204

4 Claims.

Our invention relates to direct-current generators which are used for loading internal-combustion engines, or other prime-movers, particularly those which are used for self-propelled cars and locomotives. Such generators have several special requirements, prominent among which is the ability to commutate successfully over a wide range of terminal volts and amperes.

More particularly, our invention relates to a type of power-plant using an electric generator which has a known type of excitation-system in which the main field amperes are sharply decreased as the load-amperes increase, thus causing the generator to drop its terminal-voltage sharply as the load-amperes increase. The low excitation of the main eld at high-ampere loads means that the armature-reaction is very high, in comparison with the main-eld ampere-turns, so that the main-field linx-form tends to be considerably distorted by the armature-reaction, that is, the ux tends to be greatly weakened under the leading pole-tip and strengthened under the trailing pole-tip. Of the known means for counteracting this distortion, the more commonly used curative means is a tapered main gap, which is so proportioned as to completely counteract the field-distorting effect of the armature-reaction at some chosen high-ampere load. This produces good commutation at high-ampere loads, but produces poor commutation at light-ampere loads, under which condition the tapered gap causes a main-eld-form distortion in a direction opposite to that caused by the armature-reaction.

The object of our present invention is to overcome the above-mentioned difficulties by the simple, but novel, expedient of shortening the leading main-pole tip of such a machine, that is, of a direct-current generator having an air-gap which is tapered to counteract the distortion of the armature-reaction under high-ampere load-conditions accompanied by weak main-eld excitation.

With the foregoing and other objects in view, our invention consists in the combinations, circuits, systems, machines, parts and methods of operation and assembly, hereinafter described and claimed, and illustrated in the accompanying drawing, wherein:

Fig. 1 is a curve-diagram illustrative of the general type of control which is needed on our generator;

Fig. 2 is a diagrammatic view of the generator, with a diagrammatically indicated control;

Fig. 3 is a partial sectional view of a ten-pole generator illustrative of our invention, With a conventionally indicated commutator and brushes; and

Figs. i and 5 are, respectively, a developed endview of two of the main poles and three of the cominutating poles, and the corresponding fieldform curve, of the generator shown in Figs. 2 and 3.

In Fig. l, the curve 6 is a typical Volt-ampere output-curve for the type of generator to which our invention relates, when said generator is running at full speed and receiving a certain iixed horse-power from the engine or prime-mover. In this curve, the portion 6-6" represents the constant-horse-power portion of the curve, and the operating-conditions of the generator. At loadamperes less than the value represented by the point 6', the generator is no longer capable of absorbing the given or constant output pf the prime-mover. Such a volt-ampere characteristic requires high volts at low amperes, and low volts at high amperes, with the main field strongly excited at high volts, and with a Weak main eld at high load-amperes, as shown by the fieldampere curve 1 in Fig. 1.

Fig. 2 diagrammatically indicates such a generator, in which the generator-armature and commutator are symbolically indicated by a single circle G, the armature-current being delivered by brushes B+ and B-, The field-member of the generator is provided with main field-windings F and commutating-pole windings C. The commutating-pole windings C are connected in series with the armature-circuit, which is connected to the terminal load-leads LI and L2. The main held-windings F are excited from a suitable diagrammatically indicated excitation-source E which is, or may be, responsive both to the terminal voltage of the generator and the loadcurrent of the generator, the voltage response being symbolically indicated as being obtained by the connections Il and I2 to the terminal leads LI and L2, while the current-response is symbolically indicated as being obtained from the voltage-drop across the commutating windings C, by means of the connections Il and I3. This is intended to be symbolically representative of the type of main-held excitation which is shown by the curve 'l in Fig. 1.

As more particularly shown in Figs. 3 and 4, the held-member of the generator G comprises a yoke I4 to which are attached ten main poles i5, and ten commutating poles I6. The main poles i5 have winding-receiving shank-portions l5 which are symmetrically spaced equidistantly from the commutating poles I6 or either side thereof, this equidistant spacing being necessary in order to provide room for the main-pole and commutating-pole windings F and C, respectively, as indicated in Fig. 3. The main poles I5 have pole-face portions I5 which are characterized by leading and trailing pole-tips I I and I8, respectively, which extend' out in a circumferential direction on eitherside of each shank-portion I5'. As previously mentioned, the pole-face portions I5" are tapered at the air gap, so as to compensate for the distorting eifect of the armaturereaction. The surface of the armature G is indicated, in Fig. 4, by the line I9. It will be noted that the leading pole tip I'I is spaced from the armature surface I9 by a smaller air gap than the trailing pole-tip I8.

In Fig. 5, the full-line held-form 2| shows the general shape of the resultant field-form at some specific high-ampere load, at which the elddistorting eiect of the armature-reaction, which tends. to weaken the main eld at the leading pole-tips i'I and to strengthen it at the trailing pole-tips i8, is approximately exactly counteracted by the tapering of the pole-face air gaps of the main poles, result-ing in a flat-topped fieldforin under each of the main poles,

Heretofore, the leading pole-tips i'i of generators of the type to which our invention relates have been shaped as shown in` dotted lines at Il in Fig` el,` so that the leading and trailing poletips were symmetrical with respect to the centerlines of the, main poles, with the two tips extending the same circumferential distance o-n each side of the main pole i5. Thisv had made the leading and trailing pole-tips II and I8 equidistantly spaced from the commutating poles I6 which are respectively adjacent thereto, as shown in Fig. 4.

In accordance with our invention, we space the leading pole-tips II by a greater circumferential distance from their adjacent commutating poles I6, than the corresponding spacing between the trailing pole-tips I8' and their adjacent commutating poles IG. In machines in which the available winding-space is so completely lled as to necessitate maintaining equidistant spacings between the shank-.portion I 5 of each main pole I5 and the two commutating poles in either side ci it, so as to make room for the main-eld and commutating windings F and C, the provision of an increased circumferential spacing between the leading pole-tips I1 and their adjacent commutating poles I6, is tantamount to a cutting off, or shortening, of the leading pole-.tips I'I, by an amount which is equal to the area between the dotted lines I'I and the full lines I'I in Fig. 4.

The eiect of' this shortening of the leading pole-tips i7, in accordance With our invention, is felt principally under operating-conditions in which the loading-amperes are lowA and the main field-strength is high, as shown in dotted lines in Fig.A 5. 'Under these conditions, the armaturereaction has. little distorting-effect, so that the tapered airgap causes a considerable held-form distortion, as shown by the dotted-line curve 22 in Fig. 5'. In the old type of machine, in which the field-tips I7 and I8 were symmetrical as to their circumferential extents, there was a greater fringing of flux into the commutating zone from the leading pole-.tip I'I than from the trailing tip I8, as, shown at 2 3 in Fie. 4, and as shown by the, portion b of the. dotted-line curve 22 in Fig. 5. This was due to the shorter air gap in the leading tip resulting in a shorter distance, in air, from the leading tip to the. portion of the armature-surface at the edge of the commutating zone; and this effect was considerably aggravated by the much greater ilux-density at the leading pole-tip, as shown by the slanted shape of the dotted-line held-form curve 22 in Fig. 5.

In our invention, the leading pole-tip Il" is cut baci; to the point shown by full lines at I'I, causing the corresponding fringing-lux of the iieldflux curve to be moved back to the point shown at bl in Fig. 5, so that the fringing of the mainpole flux from the leading tip I'I does not materially extend into the commutating zone at the surface of the armature. In this way, we have been enabled to` obtain good commutation over the entire ampere-range of our generator, which was impossible before the pole-tip was changed.

While we have illustrated our invention in but a single form of embodiment, we Wish it to be understood that our invention is susceptible of having various changes made, both as to the proportioningof the various parts, the substitution of equivalents, and the addition or omission of certain elements. It is desired, therefore, that the appended claims shall be accorded the broadest construction consistent with their language.

We claimas our invention:

i. A direct-current dynamo-electric machine having rnain poles and commutating poles, and windings on each set of poles, the main-pole faces being tapered along the air gap to compensate for thev distorting effect ci the armature-reaction for a given direction of rotation at. some, prede-v termined high-ampere load, thel main-pole tip which is; spaced by the smaller air gap from the armature being spaced further, from its adjacent commutating role, than the, spacing between. the other main-pole tip from its adjacent commutating pole.y

2. A direct-current generator having the ability 40 to operate over a wide range ofl terminal volts and ainperes, said generator having main poles and commutating poles, and windings on each set of poles, and means for energizing the main pole windings so. that the generator operates at high volts at low amperes and low volts at highv amperes, the main-pole faces being tapered along the air gap to compensate for the distorting effect of the armature-reaction for a given direction of rotation at some predetermined high-ampere load, the leading main-pole tip being spaced further, from its adjacent commutating pole, than the spacing between the trailing main-pole tip from its adjacent commutating pole.

3. A direct-current dynamo-electric machine having main poles and commutating poles, and windings on each set of poles, the main poles having winding-receiving shank-portions which are symmetrically spaced equidistantly from the commutating poles on either side thereof, the mainpole facesbeing tapered along the, air gap to compensate for the distorting-effect of the armaturereaction for a given direction of rotation at some predetermined high-ampere load, the main-pole tip which is spaced by the smaller air gap from the armature having a shorter circumferential extent than the other main-pole tip, whereby the fringing of flux from the shorter main-pole tip into the commutating zone is substantially prevented under light-load conditions.

4. A direct-current generator having the. 'ability to operate over a wide range of terminal volts and amiperes, said generator having main poles and commutating poles, and windings on each set of poles, and means` for energizing the, main-.pole

mutating zone is substantially prevented under light-load conditions.

CLARENCE A. ATWELL. WINSTON A. BRECHT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Kimball Mar. 18, 1941 Number 

